The present invention generally relates to an electrical filter and more particularly to an elastic surface wave filter arranged to propagate elastic surface acoustic waves (SAWs) on a substrate of a piezoelectric material for selectively deriving a predetermined frequency band region.
Recently, following a reduction in size and weight for movable communication equipment, demand for three electrode type SAW longitudinal coupling filter has been increasing, and such filters that have low loss and are superior in attenuation characteristics outside the band region are particularly required.
The following is a description of a conventional elastic surface wave filter. The fundamental type of the elastic surface wave filter generally includes a piezoelectric substrate of ST-cut crystal, 128.degree. Y-cut X propagation lithium niobate or 36.degree. Y-cut X propagation lithium tantalic acid, and each one of comb-shaped input electrode and comb-shaped output electrode provided on said piezoelectric substrate. However, the comb-shaped electrodes which effect energy conversion between electricity and an a surface acoustic wave have a bi-directional characteristic, and the energy converted from the electrical signal to the surface acoustic wave at the input electrode is propagated not only in the direction of the output electrode, but also in the reverse direction. For eliminating such an inconvenience, elastic surface wave filters of a resonator type and a multi-electrode type, etc., have been conventionally proposed.
In the resonator type, it is intended to maintain a pass-band region at a low loss by coupling the elastic surface resonator including reflectors for enclosing the surface acoustic wave disposed at opposite sides of the input and output electrodes. As a coupling means of the surface acoustic wave resonator, a multi-mode filter utilizing the longitudinal mode and transverse mode of high order has been used in practical applications. However, in the resonator type using the piezoelectric substrate, it is difficult to achieve a broad pass-band region, since the electromechanical coupling factor of the piezoelectric substrate is small, and further, freedom in design is limited due to simple electrode construction.
In the multi-electrode type, input electrodes and output electrodes are alternately disposed so as to detect the surface acoustic wave which is bi-directionally propagated by the electrodes disposed at the both sides, with another electrode being further disposed at the outer side to achieve low loss. By such a practice, if the number of repetitions of the input electrodes and output electrodes is increased, the amount of loss may be reduced to close to 0, and by a number of repetitions of 4 to 5 times, an amount of loss less than 1 dB may be theoretically obtained. However, although a large freedom in design is available in the multi-electrode type, since the total number of electrode fingers for the input and output electrodes are large, there have been problems in that influences due to multi-path reflection within the electrodes and deviation in phases between the input and output electrodes, etc., appear in the form of ripples within the pass-band region and spurious radiation outside the pass-band region, etc.
A three electrode type surface acoustic wave longitudinal coupling filter has the features of both the resonator type and the multi-electrode type, and generally includes a piezoelectric substrate of ST-cut crystal, 128.degree. Y-cut X propagation lithium niobate or 36.degree. Y-cut X propagation lithium tantalic acid, and input and output electrodes and reflectors provided on said piezoelectric substrate. However, such piezoelectric substrates as referred to above are small in the electromechanical coupling factor related to the pass-band width, and even the substrate of 128.degree. Y-cut X propagation lithium niobate having the largest electromechanical factor has a value of about 5.5%.
As is seen from FIG. 12, showing frequency characteristics of a conventional three electrode type surface acoustic wave longitudinal coupling filter constituted by a piezoelectric substrate of 36.degree. Y-cut X propagation lithium tantalic acid having an electromechanical coupling factor of 5%, regular type two input and one output electrodes, in which the number of pairs of input electrode fingers is 21, that of the output electrode fingers is 30, and reflectors of 150 pieces are respectively short-circuited and provided on said piezoelectric substrate, the ripples within the pass-band region become large as the pass-band width is broadened.
As described so far, in the conventional arrangements there has been the problem that it is difficult to maintain the practical pass-band width sufficiently large due to limitations in the number of pairs of electrode fingers affecting the frequency characteristics.